Bright red rectal bleeding typically occurs due to fissures in the anus, the passing of hard stools, hemorrhoids and persistent constipation. Sometimes, bleeding from the rectum can be due to anal or colon cancer, polyps, inflammation in the intestines, ulcers, anal fistulas and abscesses and diverticulosis, states Mayo Clinic.
An anal fissure refers to a small tear in the lining of the anus that occurs mostly due to the passing of hard stools that damages the lining. People who strain when passing stools and suffer from chronic constipation may develop hemorrhoids or swollen blood vessels in the rectum or the anus. Hemorrhoids can also occur due to pregnancy, excess body weight and anal intercourse, explains Cleveland Clinic.
Inflammation of the intestines is a less common cause of rectal bleeding and is of two types, namely, Crohn's disease and colitis. Ulcerative colitis occurs when there are ulcers in the lining of the large intestines, and ischemic colitis occurs as a result of decreased blood flow to the colon. Infectious colitis is caused by an infection to the large intestines, while radiation colitis is caused by radiation therapy administered for the treatment of cancer, reports Cleveland Clinic.
Polyps can develop into colorectal cancer and cause bleeding if left untreated. Diverticulosis, anal fistulas and perianal abscesses also lead to infections in the anal canal and consequent bleeding from the rectum, says Cleveland Clinic.